Passive oxide layers are inherent to all metal powders. In general, the presence of such oxides has an adverse effect on one or more of the properties of the products made from such powders.
For example, due to the high melting point of tantalum, its purification method yields a metal powder. When exposed to air, tantalum oxidizes and forms an oxide layer, which protects it from further oxidation. In order to make metal parts, this powder must be consolidated to solid form. Due to the inherent stability of this oxide layer, when pressed and sintered into a powder metallurgy form, the oxygen is conserved, yielding a lower quality product Therefore the oxygen removal becomes a primary objective for tantalum refining.
The operation of oxygen removal is called deoxidation. There is quite a bit of art teaching various ways of removing oxygen. One way to avoid this oxygen is to electron beam melt the powder, vaporizing the oxygen, resulting in an ingot with only the ingot's passive layer of oxygen.
A second known method for removal of oxygen from tantalum is using another element to reduce Ta2O5. One element that can be used is carbon (see, e.g., U.S. Pat. No. 6,197,082). However, since excess carbon is used for reduction, tantalum carbides result as a contaminant. U.S. Pat. No. 4,537,641 suggests using magnesium, calcium, or aluminum as the reductant (see also U.S. Pat. Nos. 5,954,856 and 6,136,062). These metals can be then leached out of the tantalum with water and diluted mineral acid, U.S. Pat. Nos. 6,261,337, 5,580,516 and 5,242,481 suggest this method for use on low surface area powders, which are used in the manufacture of solid tantalum parts. The byproduct of this process is a layer of MgO on the surface of the tantalum powder. As such it is necessary to expose this powder to air and water during the leaching and drying processes, creating the passive oxide layer. Another potential contaminant, which may result during this process, is magnesium. Magnesium tantalates are stable enough to survive the pressing and sintering processes that yield solid tantalum parts.
European Patent 1,066,899 suggests purifying tantalum powder in thermal plasma. The process was carried out at atmospheric pressure, at the temperatures exceeding the melting point of tantalum in the presence of hydrogen. The resulting powder had spherical morphology and the oxygen concentration as low as 86 ppm.
A more recent development for the removal of oxygen from tantalum is the use of atomic hydrogen as described in U.S. patent application Ser. No. 11/085,876, filed on Mar. 22, 2005. This process requires significant hydrogen excess and is thermodynamically favorable in a relatively narrow temperature range. Theoretically this process is capable of producing very low oxygen powder.
Other techniques for reducing the oxygen content of tantalum are described in U.S. Pat. No. 4,508,563 (contacting tantalum with an alkali metal halide), U.S. Pat. No. 4,722,756 (heating the tantalum under a hydrogen atmosphere in the presence of an oxygen-active metal), U.S. Pat. No. 4,964,906 (heating the tantalum under a hydrogen atmosphere in the presence of a tantalum getter metal having an initial oxygen content lower than the tantalum), U.S. Pat. No. 5,972,065 (plasma are melting using a gas mixture of helium and hydrogen), and U.S. Pat. No. 5,993,513 (leaching a deoxidized valve metal in an acid leach solution).
Other techniques for reducing the oxygen content in other metals are also known. See, e.g., U.S. Pat. Nos. 6,171,363, 6,328,927, 6,521,173, 6,558,447 and 7,067,197.
Cold spray technology is the process by which materials are deposited as a solid onto a substrate without melting. During the cold spray process, the coating particles are typically heated by carrier gas to only a few hundred degrees Celsius, and are traveling at a supersonic velocity typically in the range of 500 to 1500 meters per second prior to impact with the substrate.
The ability to cold spray different materials is determined by their ductility, the measure of a material's ability to undergo appreciable plastic deformation. The more ductile the raw materials, the better the adhesion attained during the cold-spray process due to its ability to deform.
Different metals have different plastic properties, soft metals, with excellent ductility characteristics, therefore have been used in the cold spray technology, such as copper, iron, nickel, and cobalt as well as some composites and ceramics.
In the family of refractory metals, currently only tantalum and niobium are used, as they are the softest of the refractory metals. Other refractory metals such as molybdenum, hafnium, zirconium, and particularly tungsten are considered brittle, and therefore cannot plastically deform and adhere upon impact during cold spray.
Metals with body centered cubic (BCC) and hexagonal close-packed (HCP) structures exhibit what is called a ductile-to-brittle transition temperature (DBTT). This is defined as the transition from ductile to brittle behavior with a decrease in temperature. The refractory metals, which perform poorly when cold-sprayed, exhibit a higher DBTT. The DBTT, in metals, can be impacted by its purity. Oxygen and carbon are notoriously deleterious to the ductility. Due to their surface area and affinity for oxygen and carbon, these elements tend to be particularly prevalent impurities in metal powders. Since the cold-spray process requires metals powders as a raw material, it makes the use of high DBTT refractory metals prohibitive, with the exception of tantalum and niobium, which have lower DBTT.